Sense amplifier scheme for low voltage SRAM and register files

ABSTRACT

In at least one embodiment, a sense amplifier circuit includes a pair of bit lines, a sense amplifier output, a keeper circuit, and a noise threshold control circuit. The keeper circuit is coupled to the pair of bit lines and includes an NMOS transistor coupled between a power node and a corresponding one of the pair of bit lines. The keeper circuit is sized to supply sufficient current to compensate a leakage current of the corresponding bit line and configured to maintain a voltage level of the corresponding bit line. The noise threshold control circuit is connected to the sense amplifier output and the pair of bit lines. The noise threshold control circuit comprises a half-Schmitt trigger circuit or a Schmitt trigger circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/684,842, filed Jan. 8, 2010 (now U.S. Pat. No. 8,315,119), whichclaims priority of U.S. Provisional Application 61/155,801 filed Feb.26, 2009, which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

This present application relates generally to semiconductor devices, andmore particularly to memory arrays, and even more particularly to thedesign and operation of static random access memory (SRAM) arrays and/orregister files that use single ended sensing to sense the data in a bitcell.

BACKGROUND

Static random access memory (SRAM) is commonly used in integratedcircuits. SRAM cells have the advantageous feature of holding datawithout a need for refreshing. SRAM cells may include different numbersof transistors, and are often accordingly referred to by the number oftransistors, for example, six-transistor (6T) SRAM, eight-transistor(8T) SRAM, and the like. The transistors typically form a data latch forstoring a data bit. Additional transistors may be added to control theaccess to the transistors. SRAM cells are typically arranged as an arrayhaving rows and columns. Typically, each row of the SRAM cells isconnected to a word-line, which determines whether the current SRAM cellis selected or not. Each column of the SRAM cells is connected to abit-line (or a pair of bit-lines), which is used for storing a data bitinto a selected SRAM cell or reading a stored data bit from the selectedSRAM cell.

A register file is an array of processor registers in a centralprocessing unit (CPU). Integrated circuit-based register files areusually implemented by way of fast SRAMs with multiple ports. Such SRAMsare distinguished by having dedicated read and write ports, whereasordinary multi-ported SRAMs will usually read and write through the sameports.

With the scale of integrated circuits decreasing, the operation voltagesof integrated circuits are reduced and similarly the operation voltagesof memory circuits. Accordingly, read and write margins of the SRAMcells, which are used to measure how reliable the data bits of the SRAMcells can be read from and written into, respectively, are reduced. Dueto the existence of static noise, the reduced read and write margins mayincrease the possibility of errors in the respective read and writeoperations.

For single ended sensing of a memory cell, the precharged local bit-lineeither stays at the precharged level or it is discharged to ground leveldepending on the data that is stored in the bit-cell. When the localbit-line is kept floating during the case where the cell does not havethe data value to discharge the local bit-line, the leakage from thepass gates (all cells in one column) discharges the local bit-line tozero during low frequency operation, thus making a false sensing. Toavoid this false sensing issue, the local bit-line is kept at Vddthrough a weak (small current) precharger device, i.e. a “keeper”circuit.

FIG. 1 illustrates a conventional sense amplifier circuit 100 that canbe a portion of a SRAM array or register files, and includes a keepercircuit 102. The size of components of the keeper 102 is very criticalin order to assure that the bit-cell overpowers the keeper 102 for anormal read operation. The circuit 100 is connected to bit-lines, i.e.top bit-line 108 a and bottom bit-line 108 b. The precharger 110 chargesthe local bit-line 108 a and 108 b to high state according to thecontrol signals 114 when there is no read operation. Duringmanufacturing of the memory as disclosed in FIG. 1, there are acceptablevariations in performance parameters. Process corners refer tointegrated circuits with lowest and/or highest desirable performanceparameters. Skew corners refer to integrated circuits with both lowestand highest desirable performance parameters in their sub-circuits. Atlow voltages, and skew corners (e.g. slow array transistors in bit-line108 a or 108 b and fast periphery transistors in a keeper 102), thebit-cell connected to the bit-line 108 a or 108 b will not be able tooverpower this keeper 102. Therefore, there is a limitation on thelowest desirable power supply voltage, i.e. Vdd_min, for the circuit tooperate without error.

One way to make this circuit 100 work properly under low voltage is toincrease the resistance of the keeper 102, such as increasing thechannel length of the keeper transistor 104 or decreasing the width ofthe same. This will make the keeper 102 easier to be overcome by thebit-cell connected to the bit-line 108 a or 108 b. However, this methodhas its limits due to the area that the keeper transistor 104 occupiesand also the current flow level necessary for the keeper 102 to providethe leakage current from the pass gates and thus make it operational.

Another way to make the circuit 100 operational under low voltage is tomake the trip point voltage of the NAND gate 106 higher, where the trippoint is the highest voltage where the sense amplifier output switchesfrom a high level to a low level. For that purpose, for example, whenthe NAND gate 106 comprises NMOS and PMOS, the value of β of the NANDgate 106 can be increased, where β is the ratio of Wp/Wn, and Wp and Wnare the gate widths of PMOS transistor and NMOS transistor,respectively. This ratio β determines the trip point in CMOS circuits.However, this will make the circuit 100 susceptible to noise closer tothe high state voltage because the trip point is higher. For example,when there is noise in the bit-line 108 a or 108 b close to a highstate, the output voltage could be lowered by the noise below the trippoint of the NAND gate 106, which triggers an erroneous operation.

Therefore, methods to avoid false sensing the local bit-line under lowvoltage for SRAM and/or register files are desired.

SUMMARY

In one embodiment, a sense amplifier circuit includes a pair of bitlines, a sense amplifier output, a keeper circuit, and a noise thresholdcontrol circuit. The keeper circuit is coupled to the pair of bit linesand includes an NMOS transistor coupled between a power node and acorresponding one of the pair of bit lines. The keeper circuit is sizedto supply sufficient current to compensate a leakage current of thecorresponding bit line and configured to maintain a voltage level of thecorresponding bit line. The noise threshold control circuit is connectedto the sense amplifier output and the pair of bit lines. The noisethreshold control circuit comprises a half-Schmitt trigger circuit or aSchmitt trigger circuit.

In another embodiment, a sense amplifier circuit includes a pair of bitlines, a sense amplifier output, a keeper circuit, a logic gate, and anoise threshold control circuit. The keeper circuit is coupled to thepair of bit lines and includes a first NMOS transistor coupled between afirst power node and a corresponding one of the pair of bit lines. Thekeeper circuit is sized to supply sufficient current to compensate aleakage current of the corresponding bit line and configured to maintaina voltage level of the corresponding bit line. The logic gate has inputnodes and an output node. The input nodes are coupled to the pair of bitlines, and the output node is coupled to the sense amplifier output. Thenoise threshold control circuit is connected to the sense amplifieroutput. The noise threshold control circuit is configured to havegreater driving capability to pull a voltage level of the senseamplifier output toward that of a second power node than to pull thevoltage level of the sense amplifier output toward that of the firstpower node.

In yet another embodiment, a sense amplifier circuit includes a firstdata line, a second data line, a sense amplifier output, a keepercircuit, and a noise resistant gate. The keeper circuit comprises afirst transistor and a second transistor connected in series and coupledbetween a first power node and the first data line. A gate of the firsttransistor is coupled to the sense amplifier output. The noise resistantgate comprises a first input node coupled to the first data line, asecond input node coupled to the second data line, and an output nodecoupled to the sense amplifier output. The noise resistant gate isconfigured to have greater driving capability to pull a voltage level ofthe sense amplifier output toward that of a second power node than topull the voltage level of the sense amplifier output toward that of thefirst power node.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a conventional sense amplifier circuit;

FIG. 2 is a schematic diagram of a sense amplifier circuit according toone embodiment of the present disclosure;

FIG. 3 is a schematic diagram of a half-Schmitt trigger circuit for usedas an exemplary noise threshold control circuit 206 in FIG. 2;

FIG. 4 is a graph of the output of the bit-line read/senseamplifier/read showing a trip point or the voltage at which the senseamplifier receiver switches with the same bit-line slope for (1) a priorart circuit with β=3.3, (2) a prior art circuit with β=16.7, and (3) aproposed circuit according to one embodiment of the disclosure withβ=3.3;

FIG. 5 is a graph of the output of the bit-line read/senseamplifier/read showing the bit-line slopes for (1) a prior art circuitwith β=3.3, (2) a prior art circuit with β=16.7, and (3) a proposedcircuit according to one embodiment of the disclosure with β=3.3, withdifferent bit-line voltage lines for prior art circuit and the proposedcircuit;

FIG. 6 is a schematic diagram of another embodiment of the senseamplifier circuit according to one aspect of the present disclosure;

FIG. 7A is a schematic diagram of yet another embodiment of the senseamplifier circuit according to the present disclosure;

FIG. 7B is a schematic diagram of an variation of the sense amplifiercircuit according to the embodiment depicted in FIG. 7A; and

FIG. 8 is a schematic diagram of yet another embodiment of the senseamplifier circuit according to the present disclosure.

DETAILED DESCRIPTION

The circuits of the presently preferred embodiments are discussed indetail below. It should be appreciated, however, that the presentdisclosure provides many applicable concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative, and do not limit the scope of the disclosure. Askilled person will appreciate alternative implementations.

FIG. 2 is a schematic diagram of a sense amplifier circuit 200 accordingto one embodiment of the present invention. The sense amplifier circuit200 has a keeper circuit 202. The circuit 200 is connected to bit-lines,i.e. top bit-line 208 a and bottom bit-line 208 b. The precharger 210charges the local bit-line 208 a and 208 b to a high state according tothe control signals 214 when there is no reading operation.

Further, the keeper circuit 202 has NMOS transistors 204 and a noiseresistant NAND gate 206. In this particular example, the gate node ofthe NMOS transistor 204 in the keeper circuit 202 is connected to thepower supply node and its source node is connected to the bit line. Thedrain node of the NMOS transistor 204 is connected to the power supplynode through a PMOS transistor. The NMOS 204 is only in sub-thresholduntil the bit-line read voltage reaches V_(dd)−V_(T), where V_(T) is thethreshold voltage of the transistor, thus effectively making the keepercircuit 202 weaker, i.e. easier to be overcome by the bit-line as itsvoltage decreases. In one embodiment, the noise resistant NAND gate 206(or a noise threshold control circuit) is a half-Schmitt trigger; inanother embodiment, the noise resistant NAND gate 206 is a Schmitttrigger as depicted in FIG. 2. However, in alternative embodiments,alternative circuits may be formed by rearranging the devices so thatthe β ratio is decreased or the trip point is lowered.

FIG. 3 is a schematic diagram of one example of the noise thresholdcontrol circuit 206 as indicated by NAND gate symbol in FIG. 2, using ahalf-Schmitt trigger circuit.

By lowering the trip point of the sense amplifier out, it is possible touse a lower precharge voltage level on the bit-line and avoid falsesensing of the bit-line read. The trip point is the highest voltagewhere the sense amplifier output switches from a high level to a lowlevel. The response time of the bit-line to output is reduced because ofthe improved bit-line slope of the new circuit design. The response timeof the sense amplifier output is faster due to the new scheme. Further,in at least some embodiments, the local bit-line is precharged toV_(dd)−V_(T), instead of V_(dd).

The keeper circuit 204 using NMOS transistors as shown in FIG. 2 makethe keeper circuit 202 effectively weaker, i.e. easier to overcome bythe bit-line. However, this in turn can make the prior art circuitsusceptible to noise when there are voltage fluctuations on the bit-line108 a or 108 b. To avoid the noise susceptibility, a noise thresholdcontrol circuit 206, e.g. a half-Schmitt trigger or a Schmitt triggercircuit is used in place of the prior art NAND gate 106. This schememakes it possible to perform the bit-line read operation without falsesensing at lower power voltage by having a lower trip point.

FIG. 4 is a graph of the trip point or the voltage at which the senseamplifier receiver switches with the same bit-line slope for (1) a priorart circuit with β=3.3, (2) a prior art circuit with β=16.7, and (3) aproposed circuit with β=3.3. The bit-line read plot is based on theprior art circuit 100 shown in FIG. 1. In FIG. 4, the prior art circuit100 with β=3.3 has the trip point at point (1). The prior art circuit100 with β=16.7 has the trip point at point (2). The purpose ofincreased β is to make the keeper circuit 102 weaker so that thebit-line read can overcome the keeper circuit 102 at lower power supplyvoltage. As shown in FIG. 4, the trip point (2) is higher than trippoint (1). In one circuit simulation under the power supply voltage of0.7V according to one of the embodiments, the difference is about 34 mV.However, by increasing the trip point, the sense amplifier output issusceptible to the bit line read voltage fluctuations caused by noise.This makes the prior art circuit difficult to operate at lower voltage.In comparison, the proposed circuit 200 with β=3.3 according to one ofthe embodiments has trip point at point (3). The trip point (3) is lowerthan (1) or (2). In the simulation under the power supply voltage of0.7V, the difference between (3) and (1) is about 77 mV, and thedifference between (3) and (2) is about 111 mV. This makes the proposedcircuit easier to operate at lower voltage. Also, in another simulationwith the power supply voltage of 0.6V, both sense amplifier circuitsaccording to prior art do not work at all, i.e. the sense amplifieroutputs do not switch when the bit-line voltage dropped, while theproposed circuit operates properly.

FIG. 5 is a graph of the output of the bit-line read/senseamplifier/read showing the bit-line slopes for (1) a prior art circuitwith β=3.3, (2) a prior art circuit with β=16.7, and (3) a proposedcircuit with β=3.3, with different the bit-line voltage lines for priorart circuit and the proposed circuit. FIG. 5 shows a separate bit-lineread voltage plot for the circuit 200 according to one of theembodiments. The same bit-line read voltage based on the prior artcircuit 100 shown in FIG. 1 is shown to facilitate understanding. Asshown, the prior art circuit with β=16 has a shorter response time (thetime where the trip point (2) is positioned) compared to the responsetime of point (1) of the prior art circuit 100 with β=3.3. However, theproposed circuit response time (the time where the trip point (3) ispositioned) is even shorter than the prior art with β=16.7 (the timewhere the trip point (2) is positioned). In one simulation under thepower supply voltage of 0.7V, the difference between (3) and (1) isabout 0.9 ns, while the difference between (3) and (2) is about 0.2 ns.

FIG. 6 is a schematic diagram of another embodiment of the senseamplifier circuit 600 according to the present disclosure. In thisembodiment, the NMOS 604 transistor in the keeper circuit 602 isconfigured as a diode by connecting its gate and drain node of the NMOStransistor 604. The drain node of the NMOS transistor 604 is connectedto the power supply node Vdd through a PMOS transistor 606. The sourcenode of the NMOS transistor 604 is connected to the bit line 208 aand/or 208 b.

FIG. 7A is a schematic diagram of yet another embodiment of the senseamplifier circuit 700 according to the present invention. In thisembodiment, the gate and drain node of the NMOS transistor 704 in thekeeper circuit 702 are connected to the power supply node Vdd and itssource node is connected to the bit line 208 a and/or 208 b through aPMOS transistor 706.

FIG. 7B is a schematic diagram of a variation of the sense amplifiercircuit show in FIG. 7A. In this embodiment, the gate and drain node ofthe NMOS transistor 714 in the keeper circuit 712 are connected to thepower supply node Vdd and its source node is connected to the bit line108 a and/or 108 b through a PMOS transistor 716.

FIG. 8 is a schematic diagram of yet another embodiment of the senseamplifier circuit 800 according to the present disclosure. In thisembodiment, the source node of the NMOS transistor 804 in the keepercircuit 802 is connected to the power supply node through a PMOStransistor 806 and its drain node is connected to the bit line 208 aand/or 208 b. The gate node of the NMOS transistor 804 is connected tothe power supply node Vdd. According to this embodiment, the noisethreshold control circuit 808 including strong NMOS transistors 810 areconnected in parallel to conventional NAND gate 206 to lower the trippoint of the sense amplifier output 212 by effectively lowering thevalue of β of the NAND gate 206.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example, asingle bit line circuit instead of a pair of bit-line circuit as shownin FIGS. 2-3, 6-9 can use an inverter with a single input and outputinstead of NAND gates with two inputs.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the invention described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, anydevelopment, presently existing or later to be developed, that performsubstantially the same function or achieve substantially the same resultas the corresponding embodiments described herein may be utilizedaccording to the present invention. Accordingly, the appended claims areintended to include within their scope such developments.

What is claimed is:
 1. A sense amplifier circuit comprising: a pair ofbit lines; a sense amplifier output; a keeper circuit coupled to thepair of bit lines, the keeper circuit including an NMOS transistorcoupled between a power node and a corresponding one of the pair of bitlines, wherein the keeper circuit is sized to supply sufficient currentto compensate a leakage current of the corresponding bit line andconfigured to maintain a voltage level of the corresponding bit line;and a noise threshold control circuit connected to the sense amplifieroutput and the pair of bit lines, wherein the noise threshold controlcircuit comprises a half-Schmitt trigger circuit or a Schmitt triggercircuit.
 2. The sense amplifier circuit of claim 1, wherein a drain nodeof the NMOS transistor is coupled to the power node through a PMOStransistor.
 3. The sense amplifier circuit of claim 1, wherein a gatenode of the NMOS transistor is coupled to the power node.
 4. The senseamplifier circuit of claim 3, wherein the gate node of the NMOStransistor is coupled to the power node through a PMOS transistor. 5.The sense amplifier circuit of claim 1, wherein a source node of theNMOS transistor is coupled to the corresponding bit line through a PMOStransistor.
 6. The sense amplifier circuit of claim 1, wherein a gatenode of the NMOS transistor is coupled to a source node of the NMOStransistor.
 7. The sense amplifier circuit of claim 1, wherein the gatenode of the PMOS transistor is coupled to the sense amplifier output. 8.A sense amplifier circuit comprising: a pair of bit lines; a senseamplifier output; a keeper circuit coupled to the pair of bit lines, thekeeper circuit including a first NMOS transistor coupled between a firstpower node and a corresponding one of the pair of bit lines, wherein thekeeper circuit is sized to supply sufficient current to compensate aleakage current of the corresponding bit line and configured to maintaina voltage level of the corresponding bit line; a logic gate having inputnodes coupled to the pair of bit lines and an output node coupled to thesense amplifier output; and a noise threshold control circuit connectedto the sense amplifier output, wherein the noise threshold controlcircuit is configured to have greater driving capability to pull avoltage level of the sense amplifier output toward that of a secondpower node than to pull the voltage level of the sense amplifier outputtoward that of the first power node.
 9. The sense amplifier circuit ofclaim 8, wherein the noise threshold control circuit comprises: a secondNMOS transistor coupled between the sense amplifier output and thesecond power node, the second NMOS transistor being configured to beturned off or on in response to a voltage level at the sense amplifieroutput; and a third NMOS transistor coupled between the second NMOStransistor and the second power node, and a gate of the third NMOStransistor is coupled to the keeper circuit.
 10. The sense amplifiercircuit of claim 9, wherein the first NMOS transistor is coupled to thefirst power node through a PMOS transistor, and the gate of the thirdNMOS transistor is coupled to the drain of the PMOS transistor.
 11. Thesense amplifier circuit of claim 8, wherein a gate node of the firstNMOS transistor is coupled to the first power node.
 12. The senseamplifier circuit of claim 8, wherein the gate node of the PMOStransistor is coupled to the sense amplifier output.
 13. A senseamplifier circuit comprising: a first data line; a second data line; asense amplifier output; a keeper circuit comprising a first transistorand a second transistor connected in series and coupled between a firstpower node and the first data line, a gate of the first transistor iscoupled to the sense amplifier output; and a noise resistant gatecomprising a first input node coupled to the first data line, a secondinput node coupled to the second data line, and an output node coupledto the sense amplifier output, wherein the noise resistant gate isconfigured to have greater driving capability to pull a voltage level ofthe sense amplifier output toward that of a second power node than topull the voltage level of the sense amplifier output toward that of thefirst power node.
 14. The sense amplifier circuit of claim 13, whereinthe keeper circuit further comprises: a third transistor and a fourthtransistor connected in series and coupled between the first power nodeand the second data line, a gate of the third transistor is coupled tothe sense amplifier output.
 15. The sense amplifier circuit of claim 13,wherein the first transistor is a PMOS transistor, and the secondtransistor is an NMOS transistor.
 16. The sense amplifier circuit ofclaim 15, wherein a source of the first transistor is connected to thefirst power node, a drain of the first transistor is connected to adrain of the second transistor, and a source of the second transistor isconnected to the first data line.
 17. The sense amplifier circuit ofclaim 16, wherein a gate of the second transistor is connected to thedrain of the second transistor.
 18. The sense amplifier circuit of claim16, wherein the noise resistant gate comprises: a third transistorcoupled between the sense amplifier output and the second power node,the third transistor being configured to be turned off or on in responseto the voltage level at the sense amplifier output; and a fourthtransistor coupled between the third transistor and the second powernode, and a gate of the fourth transistor is coupled to the drain of thefirst transistor.
 19. The sense amplifier circuit of claim 18, whereinthe third transistor is an NMOS transistor, and the noise resistant gatefurther comprises: an inverter having an input node connected to thesense amplifier output and an output node connected to a gate of thethird transistor.
 20. The sense amplifier circuit of claim 15, wherein adrain and a gate of the second transistor is connected to the firstpower node, a source of the second transistor is connected to a sourceof the first transistor, and a drain of the first transistor isconnected to the first data line.